It is known that allenes may be carbonylated in the presence of water or alcohols to yield carboxylic acids or esters, respectively. However, the known processes have various drawbacks which render them rather unattractive for use on a technical scale.
British Patent Specification No. 1,110,405 describes carbonylation of allene in the presence of a palladium halide and an organic phosphine. U.S. Pat. No. 3,501,518 describes carbonylation of polyolefinically unsaturated compounds in the presence of a palladium chalcogenide, an organic phosphine and an acid. The necessity of using extremely high partial pressures of carbon monoxide is a disadvantage of these known processes.
European Patent Application No. 106,379 describes carbonylation of olefinically unsaturated compounds having two double bonds in the presence of a palladium catalyst, at least 5 mol of a triarylphosphine per gram atom palladium and an acid having a pK.sub.a below 2, except hydrohalogenic and carboxylic acids. This known process has an advantage that high reaction rates are obtained at relatively low partial pressures of carbon monoxide.
It has now been found that allenically unsaturated compounds are rapidly carbonylated at relatively low pressure by using a catalytic system in which protonic acids having a pK.sub.a greater than 1.5 and/or carboxylic acids having a pK.sub.a not greater than 2.0 are combined with at least 15 mol of organic phosphine per gram atom of divalent palladium.